Various thermoplastic films have been used to package, preserve, and encase food products. Conventional compositions of films include polypropylene, polyethylene, nylon, ethylene vinyl acetate, polyvinylidene chloride, and ethylene/acrylic acid copolymer. An optional adhesive layer may be employed to combine films.
Bags made from conventional films are used in steam or hot water cooking applications, however, they are not well suited for high temperature cooking applications, such as cooking beef, because they either melt or adhere to food products retained in the bag. It is desirable to fabricate bags composed of materials that cleanly strip away from food products, such as, for example, meat products, during cooking applications. Conventional bags are also limited in their ability to be used in high pressure meat casing applications where meat is stuffed into the casing because they are subject to high elongation when heated.
It is also desirable to have bags that exhibit good mechanical properties at a low cost. The food packaging industry is constantly in search of stronger, more durable bags that are resistant to tearing and abrasion so as to completely isolate the food product from the outside environment to prevent any contaminants from entering the bag. The thickness of conventional bags is often increased at additional cost to achieve preferred mechanical properties, such as tear resistance and durability.
The food packaging industry is also searching for compositions that enable bags to be made at a lower cost. The food packaging industry attempts to reduces cost by using thin films because bags fabricated from lower thickness films typically require less materials to produce. The amount of thermoplastic elastomer used to make a film directly correlates to the overall cost of manufacturing the bag. Therefore, it is desirable to use lower thickness films to reduce the amount of material used and thus the cost of the film. It is also desired to improve the seal strengths of conventional bags in order to reduce the cost of materials. Higher seal strengths mean that a lower thickness film, and therefore less thermoplastic elastomer is required to fabricate bags.
Furthermore, it is desired that conventional bags form a barrier to gases and moisture from the outside environment. Bags that exhibit good barrier properties, such as being able to reduce or eliminate moisture and gas migration though the bag, are also desired. Oxygen migration in particular should be minimized because any oxygen that enters the bag may cause the contents of the bag, such as meat products, to spoil and/or discolor.
Sealing conventional thermoplastic films is typically accomplished by applying sufficient heat and pressure to adjacent film surfaces for a sufficient time to cause a fusion bond between the layers. However, heat sealed cylindrical bags are often imperfectly sealed because the heat necessary to seal the folds of tubular stock will harm the barrier properties of the film by either overly thinning the film layer or melting through the film. Imperfect seals are a large problem, particularly in cooking applications and when used with heat shrinkable film, because exposure to elevated temperatures and heat shrinking increases the stress on the seals. As a result, many manufacturers use ultrasonic sealing techniques or mechanical closing devices to seal conventional films.
Ultrasonic sealing techniques, however, are not desired to seal thermoplastic elastomer films, such as for example, copolyester films, because the elastic structure of the films dampens the transmission of ultrasonic energy thereby preventing the complete sealing of this type of conventional bag. Therefore, mechanical sealing devices, such as for example, clips, rings or the like, are usually preferred in place of ultrasonic sealing when thermoplastic elastomer films are used. Typically, these mechanical sealing devices are made of a plastic or metal material.
Several undesirable characteristics are associated with mechanical sealing devices. Mechanical sealing devices can become a source of contamination if lost in the food product. Metal sealing devices cannot be used in microwave ovens for cooking or defrosting of the food product contained in the bags, and the metal clips sometimes migrate into the product, thereby giving rise to customer complaints. Therefore, it is desirable to have methods of sealing thermoplastic elastomer bags that eliminate the need for a mechanical device and also resolves the sealing problems associated with heat sealed and ultrasonically sealed bags.
Therefore there is a need for improved thermoplastic elastomer films and methods of making bags from these films that overcome the drawbacks and problems associated with conventional films and methods of making bags composed of conventional films.